Perhaps 10 in. drivers have a good balance between low Fs, Vas, efficiency, power handling, cost, good upper midrange response and size to allow them to be used in groups of two, three, four, six or eight in reasonable sized enclosures.

Besides, a pair of 10s has roughly the same cone area as a single 15 so a 4x10 is, in a way, the next generation 2x15.